1. Field of the Disclosure
The present disclosure relates to a substrate having transparent electrode for a flexible display and a method of fabricating the same, and more particularly, to
2. Background
Recently, as interest in information displays has been on the rise and demand for the use of portable information media has been increased, lightweight flat panel displays (FPDs) substituting cathode ray tubes (CRTs) as existing display devices have been actively researched and commercialized.
In the FPD fields, a liquid crystal display (LCD) device, which is lighter and consumes less power, has been spotlighted so far, and recently, a development of a new display device has been actively made to meet various demands.
An organic light emitting diode (OLED) display device, one of new display devices, is a self-luminous type device, which thus is excellent in a viewing angle and contrast ratio, is lighter and thinner because it does not need a backlight, and is advantageous in terms of power consumption, relative to an LCD device. In addition, an OLED display device can be driven by a DC and at a low voltage, has a fast response speed, and is especially advantageous in terms of fabrication costs.
A thin film formed of a material having high electrical conductivity, while having transparent optical characteristics, is required to fabricate most flat panel display devices including an OLED display, and currently, a transparent conducting oxide (TCO) such as oxide indium tin oxide (ITO), indium zinc oxide (IZO), or the like, on the basis of indium oxide has been commonly used as a material of a transparent electrode.
TCO generally refers to an electroconductive metal oxide having light transmittance, which is defined as material having visible light transmittance of 80% or greater and conductivity of 10−3/Ωcm or less in a 400 nm to 700 nm wavelength region. So far, TCO has been used as an important material in flat panel displays including LCDs, OLED displays, plasma display panels (PDPs), and the like, lighting devices such as solar cells including thin film solar cells, LEDs, and the like.
ITO, one of the most widely used TCO, has various advantages such as high visible light transmission, low electric resistance, and the like. However, an increase in consumption of indium has triggered lack of resources, and a resultant increase in cost of indium and environmental problems due to toxicity of indium lead to requirement of development of a substitute material that may complement the problems.
Meanwhile, flexible displays that may be folded or rolled without being damaged are expected as novel technologies in the flat panel display fields, and in line with the development of technologies, LCDs, OLED displays, or electrophoretic display devices are anticipated to become the mainstream.
In order to apply various transparent electrodes to flexible displays, technologies of substituting ITO to enhance flexibility of thin films have been developed.
Among them, a single-walled carbon nanotube (SWNT) retains high electric charge transfer state due to very low sheet resistance and a formation of mutual networks between SWNTs.
A high length/diameter ratio of SWNT means that SWNT itself may be stretched to 100% or more, providing excellent flexibility to SWNT electrodes.
FIG. 1 is a graph showing a change in sheet resistance of an ITO electrode and an SWNT electrode over a bent angle.
Referring to FIG. 1, the SWNT electrode has sheet resistance of 103 Ω/sq or less, exhibiting excellent electrical conductivity, and has transmittance of 80% or more in a wavelength range of visible light from 400 nm to 800 nm, exhibiting excellent transmittance. For example, the SWNT electrode exhibits 80% of transmittance in a wavelength range of 550 nm and typical sheet resistance of 200Ω per square.
Here, it can be seen that the ITO electrode fabricated through sputtering has excellent electrical conductivity and transparency, but resistance thereof is sharply increased as cracks are generated due to repeated mechanical stimulation, for example, bending.
Thus, although the ITO electrode has sheet resistance superior to that of the SWNT electrode, but when the ITO electrode is folded even once, cracks are formed on the bent surface, rapidly increasing resistance, and thus, it cannot be used as a transparent electrode for a flexible display.
Also, as mentioned above, the ITO electrode involves the problem of an increase in cost due to depletion of indium and is impossible to form a pattern according to a roll-to-roll scheme to be applied to a large scale flexible display.
In comparison, the SWNT electrode and a conductive polymer electrode have excellent mechanical flexibility, but having a high sheet resistance value, and thus, it is somewhat ineligible to be applied to a large scale OLED display or a polymer solar cell.
Also, a glass substrate commonly used as a substrate of the SWNT electrode is advantageous in stability in an electrode formation or a fabrication process. However, since a glass substrate is heavy and solid, it is not appropriate for flexible displays or next-generation displays for mobile communication.
Namely, a glass substrate used as a substrate of a flat panel display is thin, having a thickness of 0.7 mm or some. However, in terms of characteristics, a glass substrate is readily broken, and when it is used as a mobile display such as a cellular phone, or the like, or applied to a large scale display, a protective window formed of glass or acryl is additionally required. Also, a glass substrate is not bent.
Various flexible substrates that may overcome such shortcomings have been developed. Among them, in case of polyimide used as a heat-resistant film, concentration of chains due to chemosystematically strong charge-transfer complex and π-conjugation reveal (or express) yellowish or brownish color, degrading transmission and transparency.